1. Field of the Invention
The present invention relates to a method for processing by laser, in which laser beam is irradiated on a processing object to directly form a fine shape on the processing object, an apparatus for processing by laser and a three-dimensional structure on which a fine shape is formed by the apparatus.
2. Description of the Prior Art
Conventionally, etching technology for using photolithography has been mainly used as a processing method that forms a highly accurate three-dimensional shape of a fine part as the processing object. In this processing method, resist material is coated on a substrate of processing object, light is selectively irradiated on the resist material, solution processing is performed, and a desired pattern is thus formed. In the method, etching is subsequently performed to remove a resist region other than a light irradiation region, and thus forming the three-dimensional shape.
In this processing method, two regions, which are a processing region, and a processed region are formed on the substrate by a series of operation from resist coating to exposure, fixing, development, development, etching of the processing object and resist removal.
However, the conventional etching processing method by photolithography has a complicated process and requires a large number of processing. Further, it also requires complicated process such as resist processing, exposure, development, baking or the like. In exposure process, a resist pattern changes due to exposure, so that it is necessary that exposure intensity (time) be controlled strictly and uniformly.
Furthermore, in forming a pattern on the processing object, a strict alignment operation is required, and its adjustment operation is required. In the case where the pattern formed on the processing object is a concave portion, a pore or the like, and its depth needs to be changed depending on a processing area, accurate alignment of exposure equipment to the substrate, to which processing has been performed once, must be performed again, and a large number of process need to be repeated.
Further, exposure conditions may change regarding exposure onto the substrate, to which processing has been made once, comparing to exposure onto an unprocessed flat substrate. Moreover, in the processing by photolithography, a method to form the resist pattern by light is mainly used. In this case, it is difficult to define a material-removing region smaller than an irradiation region of light on the processing object.
In addition, in a conventional microfabrication processing by laser, carbon dioxide gas laser (CO2 laser), ultraviolet laser represented by excimer laser, short pulse laser, or second and third harmonics of three-dimensional-state laser such as Nd:YAG, Nd:YLF and Nd:YVO4 has been used.
The excimer laser has a wavelength from 157 nm to 309 nm and a pulse width from a few nanoseconds to a few tens nanoseconds as central values. The excimer laser is used particularly for polymer having high absorption to light within the wavelength range. Since the processing method by the excimer laser can remove matter on a laser irradiation area (laser irradiation region) by laser of extremely short pulse comparing to its thermal diffusion length, it is known as a method having high accuracy of the processing region and little thermal damage.
Furthermore, a processing method using femtosecond laser is known in recent years as a microfabrication processing method for metal or the like. This is a processing method using laser beam having the pulse width from a few tens femtoseconds to a few hundred femtoseconds as the central value. There exists Ti: Sapphire laser as a typical light source used in this processing method. These are known to be capable of performing fine and highly accurate processing to various kinds of material such as metal and ceramic. The processing method by the laser is described in the thesis shown below.
Appl. Phys. A 63, pp 109-115 (1996)
Applied Physics 67(9), p 1051 (1998)
O Plus E 21(9), p 1130 (1999)
Since the processing of transparent material by the laser basically uses multi-photon absorption of material, it is said to be capable of processing an object having a size of the wavelength or less. However, laser processing condition is required that the intensity of light be relatively large in processing the transparent material.
Further, there is known a metal removal method for correction by photomask in processing thin film layer by conventional laser. This is a method where laser beam is irradiated from above a metal thin film deposited on a glass substrate, and the metal film is selectively removed by fusing and vaporizing it.
However, processing by CO2 laser or YAG laser is basically thermal processing. For this reason, when a micro region is processed and formed on the processing object by using the laser, the vicinity of an irradiation region of laser beam easily causes thermal deformation and fusion in low melting point material such as polymer, so that highly accurate processing is difficult.
Further, in brittle material such as glass and ceramic, there are cases where fracture or crack occurs in the irradiation region of laser beam, it has been difficult to form a fine shape on the processing object.
Furthermore, in the processing by harmonic of excimer laser or YAG laser, a processing range is generally defined only to material having high absorption to its laser wavelength, and thus the processing of the transparent material has been difficult, or it has been necessary that the material be processed by vacuum ultraviolet beam that cannot be used in the atmosphere. Even in such a case, the processing has often required extremely high irradiation intensity of laser beam.
Moreover, it has been difficult to form a region smaller than the irradiation region of laser beam on the processing object even if the laser beam for processing is used, and thermal diffusion or the like sometimes has caused a processing region to have a larger width than the irradiation region of laser beam.
In the case of ablation processing by Ti: Sapphire laser having the pulse width from a sub-pico second to a pico second, it is known to be capable of performing highly accurate processing even to the metal material. However, the transparent material has a long light permeability length and removal of micro amount of matter of the processing object has been difficult, and there have been cases where crack or the like occurred.
In recent years, micropore processing by the multi-photon absorption of laser in the sub-pico second region has been implemented. In this processing method, laser beam is focused into high intensity to efficiently perform light absorption of a transparent body as the processing object. This makes it possible to process and form the micro region on the processing object by resolving power of the wavelength or less. However, in this case, there has existed a problem that extremely large energy density was required and a laser light source with high energy in an extremely short pulse width was required.
Further, although it is possible to remove the matter of thin film layer formed on an upper surface of the processing object by a thin film processing method using a conventional laser ablation method, no consideration is paid to removing the matter on a surface of the processing object, which exists under the thin film layer, and forming a fine shape on a surface of processing object.
Still further, a method of simultaneously removing the matter of the thin film layer and on the surface (contact surface or boundary surface) of the processing object by using the thin film layer, and a processing method of reforming adjacent material by utilizing absorption of the thin film have been suggested. However, technology regarding fine structure processing of the laser irradiation region or smaller has not been known.